1. Field of the Invention
This invention relates to liquid purification or separation and, more particularly, to the deposition of oriented monolayers on the surface of separation membranes.
2. Description of the Prior Art
The cost and energy effectiveness of membrane separation processes are seriously compromised by the readiness with which available membranes undergo fouling by colloidal materials. The anion-exchange membranes of electrodialysis (ED) stacks and the uncharged membranes of reverse-osmosis (RO) systems are especially prone to fouling.
The heart of every modern electrodialytic treatment system is an alternating array of polymer-based ion-exchange membranes. A serious obstacle to cost-effective operation of electrolytic desalination plants is the ease with which these membranes undergo concentration polarization and fouling by humic acids. These end products of biodegradation are present in most natural waters as colloidal materials bearing partially ionized acid groups. Their negative character renders them much more likely to adhere to a positively charged anion-exchange membrane than to a cation-exchange membrane, and this adherence has two deleterious effects: first, the pores of the membrane become physically occluded by colloidal material; and second, the positive bulk with a negative fouled surface functions as a bipolar "sandwich membrane", greatly enhancing its tendency to undergo further fouling.
During continuous operation, these insoluble impurities occlude the membrane surfaces at an increasing rate, and the electrical resistance of a stack is raised to the point where power costs make further operation uneconomic. The stack must then be disassembled for stringent cleaning or replacement of membranes. The combined expenses of down-time, replacement, or cleaning, and power requirements that rise steadily during operation seriously compromise the cost effectiveness of this method of water purification.
Fouling of RO membranes proceeds by a less well known, but related pattern, in which colloidal materials are occluded on the working surfaces of the membranes almost immediately after operation has been initiated. In RO separations, greatly reduced membrane flux is the negative economic factor.
Considerable evidence indicates that a propensity toward polarization and fouling is governed by the nature of a membrane surface. Critical surface characteristics have been shown to include rugosity (roughness), chemical homogeneity, and hydrophilicity. Their demonstrated importance indicates that surface modification may offer a fruitful avenue to a mechanistic definition of concentration polarization and fouling, and to their mitigation.